JP2964838B2 - Wavefront shaping device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavefront shaping device for shaping the wavefront of a light source such as an interferometer and the wavefront of reference light of an interferometer.

[0002]

2. Description of the Related Art Conventionally, a wavefront shaping device has been used for shaping a light source wavefront and a reference wavefront used for an interferometer or the like.

FIG. 7 shows a conventional example, in which an incident beam is generally a disturbed plane wave like a wavefront 10. The disturbed wavefront 10 is focused on the pinhole 701 by the focusing lens 7. The intensity distribution on the pinhole 701 has a Gaussian spread as shown by a curve 218 as shown in FIG. This is because the spot has a distribution on the pinhole 701 surface because the wavefront 10 is disturbed and is not a simple plane wave.

The pinhole 701 plays a role in shaping the wavefront. The diameter of the pinhole 701 is determined by the NA of the collimator collimator lens 9 so that the wavefront after the collimator lens 9 becomes a perfect plane wave. It is set to be about the same as the limit. This is shown in FIG. Generally, the pinhole diameter 801 is smaller than the spread of the intensity distribution 218 on the pinhole surface, and the pinhole 701 blocks light.

As a result, the pinhole 701 functions as a point light source for the collimator lens 9, and the light emitted from the collimator lens 9 becomes a complete plane wave 11. A system including the condenser lens 7, the pinhole 701, and the collimator lens 9 is called a wavefront shaping device.

[0006]

However, there is a problem in the adjustment of the optical system in the conventional example. In a general arrangement, the diameter 801 of the pinhole 701 is about 1 μm, and the diameter of a beam focused there is often several μm. Therefore, it is difficult to align the center of the pinhole 701 with the center of the beam.

Another problem is the concentration of energy. When shaping the wavefront of a laser having a high energy per unit time, such as an excimer laser, as the laser, the energy on the pinhole surface located near the focal point of the luminous flux is too high and the pinning speed is short. The hole will be damaged.

[0008]

According to the present invention, at least one etalon is provided before a condenser lens in a wavefront shaping device comprising a condenser lens, a pinhole, and a collimator lens. The diameter of the pinhole, which is a spatial filter, is set to a diameter that allows only one order to pass through among conditions that are strengthened by multiple interference inside the etalon.

As a result, only a single order propagating in the same direction under the interference conditions of the etalon is made parallel by the collimator lens, so that a complete plane wave can be obtained as in the conventional example.

This method does not cut off unnecessary light rays by a pinhole as in the prior art, but cuts most of the unnecessary light rays other than the desired order by an etalon. Therefore, the energy density can be reduced before the light beam is condensed, the energy directly hitting the pinhole surface is reduced, and the pinhole can be prevented from being damaged. Further, since the diameter of the pinhole may be set to a condition at which at least the order next to the order to be transmitted is cut, the diameter of the pinhole can be made larger than in the related art, and the conditions for alignment are greatly eased.

[0011]

FIG. 1 shows a first embodiment of the present invention. The difference from the conventional example of FIG. 7 is that two etalons 1 and 2 are provided before the condenser lens 7.
And the diameters of the pinholes are different.

The incident wavefront 10 is not a perfect plane wave as in the prior art, but a disturbed wavefront. In particular, in the case of an excimer laser, since the transverse mode is multimode, the wavefront is significantly disturbed. The wavefront 10 first enters an etalon 1 (gap thickness d ₁ ) composed of two parallel flat plates 3 and 4 provided with a highly reflective film. Assuming that the incident angle on the etalon is θ, the light beam is transmitted only in the direction θ that satisfies 2nd ₁ cos θ = m ₁ λ (1). Here, n is the refractive index between the parallel plates, λ is the wavelength, and m ₁ is the order of interference.

FIG. 2 shows the incident angle θ on the horizontal axis and the transmitted light intensity on the vertical axis. As shown in FIG. 2, light rays are transmitted only in discrete directions of intensity distributions 201 to 209. I do.

Next, the light beam is incident on an etalon 2 (gap thickness d ₂ ) composed of two parallel flat plates 5 and 6 provided with a highly reflective film, and this time, 2nd ₂ cos θ = m ₂ λ... Light rays in the direction satisfying (2), that is, the intensity distributions 210 to 2 in FIG.
Light rays are transmitted only in 16 discrete directions. However, in (2), m ₂ indicates the order of interference.

Depending on how to select the gap thicknesses d ₁ and d ₂ ,
In this embodiment, the intensity distributions 202 and 211 and the intensity distribution 205
213 and intensity distributions 208 and 215 are common.

Therefore, the light transmitted through the etalons 1 and 2 is a common part between them, that is, the intensity distributions 202, 205, and 208 in FIG.
And enters the condenser lens 7 only. FIG. 1 shows a part of the rays, and the light incident on the condenser lens 7 is condensed on the pinhole surface 8 as shown by the transmitted rays 12 and 13. Focusing position x on pinhole surface 8
Is expressed as x = ftan θ where f is the focal length of the condenser lens 7.

Therefore, the light beams transmitted through the etalons 1 and 2 are located at positions 301, 302 and 3 on the pinhole 8 as shown in FIG.
Focus on only 03. Position (light flux) 30 in FIG.
Reference numerals 1 and 302 correspond to the intensity distributions 202 and 205 in FIG. 2 and the luminous fluxes 12 and 13 in FIG.

Here, the intensity distributions 205 and 213 have θ = 0,
That is, it corresponds to the light beam 302 propagating in the direction coinciding with the optical axis of the condenser lens 7.

Therefore, as shown in FIG.
If a pinhole having a diameter 304 that blocks the light beam 303 is arranged, only the on-axis light beam focused on the light beam 302 is collimated.
The light can be guided to the lens 9.

Here, considering the angular distribution of the light beam 302 based on the transmission characteristics of the etalon, the half-value width Δδ as the phase of the transmitted light at θ = 0 deg is

[0021]

(Equation 1) It is expressed as

Here, F is an amount called a finesse, and is an amount defined by the following equation (5) when the intensity reflectance of the etalon high reflection film is r ² .

F = 4r ² / (1-r ² ) (5) The phase at the peak in FIG. 2 is δ ₀ , and the transmission angle at this time is θ
₀ , if the phase at the half value is δ ₁ and the transmission angle at this time is θ ₁ , δ ₀ = (2π / λ) 2nd cos θ ₀ (6) δ ₁ = (2π / λ) 2nd cos θ ₁ (7) From the relationship of the half width, δ ₀ −δ ₁ = Δδ / 2 (8) From the expression (4), (6) and (7),
If you assign to, θ ₁ is

[0024]

(Equation 2) Accordingly, the half-value width Δx on the pinhole surface in FIG. 3 is represented by Δx = 2f tan θ ₁ (10) from (3) and (9).

For example, d ₁ = 9999.980 μm, d ₂ = 6840.0
Assuming that 88 μm and r = 99%, θ = 0 in the equations (1) and (2) is m ₁ = 80645 and m ₂ = 55162 when the wavelength λ is 0.248 μm.

At this time, the orders adjacent to m ₁ and m ₂ , that is, m ₁ -1, m ₂ -2,... And m ₁ -1, m ₂ -2
The focal position x that satisfies the following conditions: The focal length f is 10 mm
Then, it can be shown in the following table.

[0027]

[Table 1] Therefore, in the orders shown in this table, the common parts θ = 0 deg, x = 0 mm θ = 0.494 deg, x = 0.0962 mm and θ = 0.494 deg,
Only the overlapped portion of x = 0.862 mm is a light beam that passes through etalons 1 and 2.

The half width Δx is as shown in the following table from (10).

[0029]

[Table 2] For example, when NA = 0.1, the diameter of the diffraction limit is 0.61 λ / NA =
1.5 μm, half width of etalon 1 when r = 0.99 1.9μ
less than m. However, in practice, the half value width of the transmitted light is a product of the distributions of etalons 1 and 2, so it is sufficient to set r ≧ 99%.

Therefore, the focal length f of the condenser lens 7 is given by f =
When the distance is 10 mm, the focal position x on the pinhole 8 is
If the axis is assumed to be 0, it is expressed as x = f tan θ, so x = 0 mm when θ = 0 deg x = 0.0862 mm when θ = 0.494 deg x = −0.0862 mm when θ = −0.494 deg . Φ = 0.08mm, considering the spread due to half width
With this arrangement, only the light beam of θ = 0 deg and x = 0 mm can be guided to the collimator-lens 9.

Taking the above numerical examples, a system in which the beam diameter on the pinhole is about φ1.5 μm with respect to the pinhole diameter φ = 80 μm in the configuration of FIG. 1 is realized. Conventional 1μm
It can be seen that the alignment according to the present invention is greatly facilitated as compared with the case where the pinholes having the front and rear diameters are used. The transmitted light at θ = 0 ° does not hit the pinhole plate, and the transmitted light at θ = ± 0.494 ° is shown in FIG.
As shown in (1), it corresponds to a weak portion around the beam. Therefore, the pinhole plate is not damaged.

In the conventional example, the light focused on the pinhole is 2
It is reflected by two etalons and does not reach the pinhole. As a result, without having to struggle with the alignment of the pinhole plate, and without damaging the pinhole plate,
A complete plane wave 11 can be obtained.

The wavefront shaping device using the etalon having such a configuration is particularly effective in an interferometer constituted by a light source having a high energy per unit time such as an excimer laser. In this case, the wavefront shaping device is arranged in the light source section in the interferometer for shaping the wavefront of the light source into a plane wave, or is arranged in the reference optical path for shaping the reference wavefront. However, the effect of the etalon according to the present invention is fundamentally different from the wavelength selection effect of an etalon provided in a resonator such as an excimer laser.

This is because, in the present invention, the transverse mode of the laser is selected by the etalon and the spatial filter, whereas the etalon in the laser resonator selects the longitudinal mode for narrowing the wavelength band.

Therefore, the etalon used in the present invention is completely different from the etalon used in the resonator in the set gap thickness. In fact, the etalon alone for the longitudinal mode selection in the laser cannot purely select the transverse mode.

FIG. 4 shows a second embodiment of the present invention. The etalons 1 and 2 of the first embodiment have tilt adjusting mechanisms 401 and 402 respectively.
Is provided. In the embodiment of FIG. 1, in order to set the transmitted light of the etalons 1 and 2 to a desired angle, the gap interval d ₁ ,
It is necessary to create d ₂ with a predetermined value with high accuracy. This is an essential condition for aligning the propagation directions of the two etalons, but there is a problem in assembling because high accuracy is required for setting the gap.

However, as shown in FIG.
By adding tilt adjustment mechanisms 401 and 402 to
That problem is solved. As can be seen from Equations (1) and (2), similarly to the adjustment of the gap thicknesses d ₁ and d ₂ , the adjustment of the incident angle θ has the same effect on the phase.

Therefore, by adjusting θ, the gap thicknesses d ₁ , d
Light rays can be transmitted at a desired angle except for the error of ₂ . Thus, the transmitted light can be obtained at a desired angle by a simpler method of adjusting the inclination of the etalons 1 and 2 without adjusting the gap thicknesses d ₁ and d ₂ .

FIG. 5 shows a third embodiment of the present invention. In the present embodiment, the pinhole 501 is arranged at a position defocused from the focal position of the condenser lens 7, and the diameter of the pinhole 604 is such that adjacent orders of light are not mixed as shown in FIG. And a pinhole 501.

As shown in FIG. 6, the transmitted light 601, 602, 603 of the etalons 1, 2 reduces the energy density on the surface of the pinhole 501 due to the effect of defocus, thereby preventing the pinhole 501 from being damaged. Thus, an improvement in durability can be achieved. However, in this embodiment, as can be seen from FIG. 6, compared with the first embodiment shown in FIG.
The beam diameter of No. 2 expands, and the pinhole diameter 604 decreases. For this reason, the alignment of the pinhole 501 becomes slightly difficult, but the effect of improvement over the conventional example is apparent.

Therefore, this embodiment is particularly effective for a system having a sufficient degree of difficulty in alignment. In the embodiments described above, the effect of order selection is emphasized by using two etalons. However, it is possible to separate orders by using only one etalon, and the effect of the present invention can be sufficiently achieved.

[0042]

As described above, in the present invention, the diameter of the pinhole can be increased by adding the etalon to the conventional wavefront shaping device, so that the alignment becomes easy and the unnecessary light is further reduced. Can be reflected by the etalon and removed, thereby preventing the pinhole plate from being damaged by the beam. As a result, it has become possible to configure a wavefront shaping device that is easier to adjust than conventional and has excellent durability.

[Brief description of the drawings]

FIG. 1 is a diagram showing a first embodiment of the present invention.

FIG. 2 is a diagram showing transmission angle characteristics of each of two sets of etalons.

FIG. 3 is a diagram showing synthesized transmission characteristics of two sets of etalons.

FIG. 4 is a diagram showing a second embodiment of the present invention.

FIG. 5 is a diagram showing a third embodiment of the present invention.

FIG. 6 is a diagram showing an intensity distribution on a pinhole surface according to a third embodiment.

FIG. 7 shows a conventional example.

FIG. 8 is a diagram showing an intensity distribution of a condensed beam near a pinhole in a conventional example.

[Explanation of symbols]

1, 2 etalon 3, 4, 5, 6 parallel plate with high reflection film 7 condensing lens 8 pinhole 9 collimator lens 10 disturbed incident wavefront 11 plane wave 12 incident angle other than 0 deg transmitting through etalons 1 and 2 13 A light beam having an incident angle of 0 deg that transmits etalons 1 and 2 201 to 209 A component that transmits etalon 1 210 to 216 A component that transmits etalon 2 2170 deg Half-value angle of transmitted light 218 Peak of etalon transmitted light 301 to 303 A component that transmits both etalon 1 and etalon 2 304 Pinhole diameter 3050 deg Half angle of transmitted light 401, 402 Etalon tilt adjustment mechanism 501 Pinhole 601 to 603 Beam on pinhole surface Strength distribution 604 Pinhole diameter 701 Pinhole 801 Pinhole diameter

Claims (3)

(57) [Claims] At least one etalon, a condenser lens, a spatial filter disposed near a focal position of the condenser lens, and a collimator for passing light passing through the spatial filter.
A wavefront shaping device, comprising: 2. The etalon allows at least one or more light beams in a plurality of directions determined by the order of the interference condition to pass therethrough, and the spatial filter has a light beam in a direction corresponding to only one of the light beams in the plurality of directions. Through,
2. The wavefront shaping device according to claim 1, wherein light other than said order is not transmitted. 3. The wavefront shaping device according to claim 2, wherein the etalon has a tilt adjusting mechanism.